An image forming apparatus such as a printer or copying machine has a device for conveying a paper sheet to discharge it after an image is formed thereon. In some cases, however, the timing of feeding paper sheets varies to result in variations in the intervals between the respective paper sheets when they are discharged.
Japanese Patent Laid-Open No. 11-59965 discloses a technique of detecting variations in paper sheet intervals, expected to occur at the time of paper feed, at a transfer unit, and automatically correcting them to bring the paper sheet intervals near to desired paper sheet intervals.
Japanese Patent Laid-Open No. 2001-240285 discloses a technique of preventing the occurrence of a conveyance error such as a jam by detecting the interval between paper sheets on a reverse route, when each paper sheet is reversed and discharged to be registered, and correcting the interval to a predetermined value.
In the above technique of detecting the intervals of paper sheets, a reflection type photosensor and a sensor for detecting the presence/absence of a paper sheet that passes are used to detect the interval between the trailing end of a preceding paper sheet and the leading end of a succeeding paper sheet, and the motor is controlled to change the convey speed, thereby bringing the interval to a desired interval.
According to this technique, the intervals between paper sheets can be corrected by detecting the intervals of the respective paper sheets and changing the convey speed of paper sheets. If, however, the interval between paper sheets deviates, since the convey speed of subsequent paper sheets is changed, the intervals between the respective paper sheets deviate after the change in convey speed. More specifically, if the interval between the jth (j is an arbitrary positive number) paper sheet and the (j+1)th paper sheet is shorter than a desired interval, the convey speed of the (j+1)th paper sheet must be decreased to increase this interval. Even if, however, the interval between the (j+1)th paper sheet and the (j+2)th paper sheet has been normal before this control, this interval decreases after the control. This time, therefore, the convey speed of the (j+2) th paper sheet must be decreased. As described above, once the interval between any two paper sheets deviates, the convey speeds of the succeeding paper sheets must be changed, resulting in complicated control.